Harry Saville is a Deputy Chairman of Aberconwy Conservative Association and interns with a Member of Parliament.

The majority of the UK’s main political parties have publicly decided that the UK’s nuclear deterrent will continue to be based solely around the Trident II D5 ballistic missile. What remains unanswered is what kind of posture the UK’s nuclear forces will adopt in the future.

The “Main Gate” procurement decision for the successor programme to replace the current Vanguard fleet is due in 2016. The decision to procure a two, three or four boat fleet will have profound affects on what sort of posture the UK’s nuclear forces adopt in the future. Now is the time to reflect upon the requirements of our nuclear deterrent, and to make a case for the future of the UK’s nuclear deterrent that goes beyond the soundbites of “ultimate insurance” and “part-time deterrent”.

The most demanding role placed upon the UK’s nuclear forces is also its main rationale – the provision of a “second centre of decision” within NATO. When Harold Macmillan and John F Kennedy negotiated the Polaris sales agreement (which today provides for the independent UK deployment and use of US supplied Trident missiles), it was agreed that this part of the UK’s nuclear deterrent would be assigned to NATO, unless the Government decides that supreme national interests are at stake.

In spite of some opposition within his own government, Kennedy entered into this agreement in the knowledge that a second credible, independent nuclear deterrent within NATO supported the security guarantee otherwise exclusively provided by the USA.

Today, the USA and UK are joined by France in shouldering NATO’s strategic nuclear burden, but the dynamic remains the same. Any aggressor attacking a NATO member state who believes the USA might not back up its nuclear guarantee in an emergency has to consider the UK’s response as well. Whilst a US president might not be prepared to risk retaliatory nuclear strikes on New York, Chicago and Washington over a regional conflict in Europe, a British Prime Minister, faced with a threat much closer to home, might still be prepared to resort to nuclear strikes. This “second centre of decision” which serves to complicate the decision making progress of any aggressor, is considered a huge benefit to NATO.

The last National Security Strategy identified NATO-relevant threats to which the UK may be expected to respond with nuclear strikes. Broadly speaking, they are a large scale conventional attacks or attacks involving chemical, biological, radiological or nuclear weapons against the UK or a NATO member state.

There are a handful of states that could potentially threaten such an attack – Russia, Iran and China being examples. However, Russia stands head and shoulders above the rest. Its strategic aims over the coming decades are unclear. Over recent years, it has shown a willingness to use force to promote its agenda within Europe. This has ranged from nuclear bomber patrols probing UK, Norwegian and US airspace to active military deployments in Georgia and Ukraine (in the case of the latter, to cement its future as a Russian client state and weaken its links with NATO and the EU).

Whilst the British Government does not specifically label Russia as a nuclear threat for diplomatic reasons, it is a benchmark against which the UK’s nuclear strategy should be evaluated. Whilst other states do not pose such a threat at present, their ability to potentially to pose a greater threat in the future cannot be discounted.

Since adopting submarine-launched ballistic missile weapons systems the UK has maintained a Continuous At Sea Deterrence, [CASD] posture. In 1985, when the UK’s deterrent was provided by a four boat fleet of Resolution-class ballistic missile submarines (SSBNs) armed with sixteen Polaris ballistic missiles (SLBMs), CASD meant ensured one boat one station ready to fire at minutes notice 20 per cent of the time, two boats 73 per cent of the time and three boats seven per cent of the time.

With Polaris SLBMs carrying multiple warheads, the UK was able to strike up to 32 targets with a nuclear salvo the majority of the time. 2010’s Strategic Defence and Security Review (SDDR) placed restrictions on the deployment of Trident. The UK is limited to deploying one submarine at any on time, armed with a maximum of 40 warheads mounted on eight missiles. Whilst the warheads carried by Trident are estimated to be less powerful than Polaris, the capability to independently target them provides the capability to strike up to 40 targets with only one boat on patrol. Despite operating the minimum nuclear deterrent, this gives the UK the most flexible targeting options available.

Russia holds its nuclear forces at high readiness. Evidence suggests that it envisages its nuclear forces playing an active role in any large-scale military engagements. Therefore, the UK must be prepared to contribute to a second, non-US, continuous nuclear deterrent for NATO.

This leaves the UK with the option of procuring either three or four SSBNs. The four boat option will ensure there are at least two SSBNs available for patrol at any one time. The three boat option, depending on the design characteristics of the Successor SSBN, will ensure one SSBN on patrol at any one time.

With the three boat option there might be periods where there is no reserve SSBN available. February and March 2009 saw continuous patrols carried out with two SSBNs, whilst HMS Vigilant was in mid-life overhaul and HMS Vanguard was under repair. Between 1996 and 1998, continuous patrols were provided by two boats, HMS Vanguard and HMS Victorious without any backup. New technology incorporated into the successor boats should increase the capabilities of the new SSBNs. To continue to provide a credible nuclear deterrent for NATO, at the “Main Gate” the UK must commit to building three, if not four, new SSBNs.